In view of the fact that a contact lens generally must allow the cornea to be supplied with oxygen, the soft contact lens usually has been formed from a suitable hydrophilic monomer, such as hydroxyethylmethacrylate, so as to improve the oxygen permeability, while the hard contact lens conventionally has been formed from hydrophobic material of high oxygen permeability or the lens size has been reduced and, in this case, lacrima has been relied for the oxygen supply.
The soft contact lens is certainly advantageous in that, during its use, the resinous material constituting this contact lens is sufficiently swollen with moisture to facilitate the oxygen diffusion and such resinous material of soft type eliminates or reduces a sense of incompatibility, allowing the user to wear the contact lens for a long period. Thus, the contact lens of this type has been widely accepted. However, the contact lens of this type has intrinsic disadvantages with respect to mechanical strength such as tensile strength and tear strength as well as with respect to a correspondingly limited durability. Additionally, the soft contact lens is made of hydrophilic material to which undesired substances such as bacteria or protein readily cling and this necessarily leads to requirement for troublesome periodical treatment of the contact lens, e.g., boiling or sterilizing treatment.
Concerning the hard contact lens, on the other hand, there has recently been available a contact lens utilizing silicone resin having a high oxygen permeability coefficient, but it cannot be expected for such contact lens to effect an adequate oxygen supply to cornea.
Both the soft and hard contact lenses which are presently available have an oxygen permeability coefficient as low as 1 to 10.times.10.sup.-11 cm.sup.3 .multidot.cm/cm.sup.2 .multidot.sec.multidot.mmHg and, at such level of oxygen permeability, it is impossible for the user to wear the contact lens for a long period. Instead, the user is normally obliged to take his or her contact lens off every day or every few days and to be diagnosed by an ophthalmologist.
Making the contact lens porous in a physical manner already has been attempted by subjecting the contact lens to various kinds of processing, e.g., electrical discharge and laser irradiation, in order to improve the oxygen permeability of the contact lens. However, it has been difficult for such prior art processing to form fine pores each having a sufficiently small diameter in the order of angstroms to pass only gaseous molecules, such as oxygen molecules, therethrough and always resulted in forming pores of 500 .mu.m.phi. or larger. Furthermore, such prior art processing inconveniently has led to a transparency loss of the resin and resulted in the optical material unsuitable for ophthalmologic application.
It is well known to form porous film used as separating film by subjecting high molecular resin film to irradiation of ion beam and then subjecting this to etching. Because the separating film, in view of the purpose of its use, is not required to be transparent, the etching has usually been performed at a temperature of 180.degree. C. or higher, utilizing strong alkali or strong acid also of a high temperature. Accordingly, the film obtained in this manner is certainly porous but opaque, since the initial transparency of the high molecular resin film has been lost in the course of said processing. The contact lens used for ophthalmologic purpose, on the contary, must be provided with pores each having a diameter within a predetermined range at a pore density also within a predetermined range, without a significant transparency loss. More specifically, each pore must have a size precisely restricted so as to pass only the oxygen molecules therethrough and a pore size in excess of such critical limit might often facilitate protein content of lacrima to be mixed with various bacteria which, in turn, might proliferate and cause eye diseases.
A primary object of the present invention is to provide soft and hard contact lenses which have an oxygen permeability sufficiently improved for continuous wearing for a long period, and which can be easily handled from a hygienic viewpoint, and thereby eliminate the above-mentioned drawbacks encountered by the oxygen permeable contact lenses of well known art.